1. Field of the Invention
The present invention relates to an image display device. In particular, the present invention relates to an image display device capable of displaying three-dimensional images to a viewer wearing polarizing glasses by utilizing the viewer's interocular parallax, and capable of displaying high-resolution two-dimensional images to a viewer not wearing such polarizing glasses, the polarizing glasses being inexpensive and lightweight.
2. Description of the Related Art
There is a relatively long history of attempts at reproducing three-dimensional (3D) or stereoscopic images, which resulted in a vast variety of systems including laser holograms and the like. However, only a few can display 3D motion images in full color at a practically acceptable level. Examples of such systems include methods (1) to (3) described below (all of these systems are based on the principle of separately displaying an image for the right eye and an image for the left eye so as to evoke the illusion of a "depth" of the displayed image to a viewer, the difference in position between the image for the right eye and the image for the left eye being called an "interocular parallax"):
(1) A shutter glass method. A single display device is employed to alternately display an image for the right eye and an image for the left eye on a time-division basis, while a viewer watches the images through special glasses having electrical shutters which alternately open or close in synchronization with the displayed images, thereby displaying a 3D image. This method is applicable to both projection displays and direct-view type displays.
(2) An autostereoscopic method (i.e., a method which does not require the use of any special glasses). Stripes of an image for the right eye and an image for the left eye, displayed on an image display device, are respectively allocated to the right eye and the left eye by means of a lenticular lens plate or a slit plate located in front of the display device. Under this system, a viewer can observe a 3D image without wearing any specially-designed glasses or the like.
(3) A polarizing glasses method. A 3D image is displayed by supplying a linearly-polarized image for the right eye and a linearly-polarized image for the left eye to a viewer wearing polarizing glasses, the directions of polarization of the glasses constituting 90.degree. with respect to each other. In the case of a projection display, this method typically employs two polarizing projectors for superimposing the respective images on each other on a display screen. In the case of a direct-viewing type display, this method employs two display devices, whose displayed images are synthesized by means of a half mirror or a polarizing mirror.
Method (1) described above provides an advantage of requiring only one display device to display a 3D image. However, it has a disadvantage in that the viewer is required to wear special glasses (e.g., liquid crystal shutter glasses) which are capable of electrically shutting or opening the view. Such glasses inevitably become heavy and therefore tend to cause fatigue to the viewer wearing them over a long period of time. Moreover, such glasses are costly. Given that one pair of such glasses is required for each viewer, the total expense for a number of viewers may become very high.
Method (2) provides an advantage in that the viewer can observe a 3D image without wearing any special glasses or the like. However, it is disadvantageous in that the zone within which the viewer is able to observe a 3D image view is very narrow. The reasons for this problem are described below, with reference to FIGS. 23 and 24 (illustrating a case where a lenticular lens plate is employed).
FIG. 23 illustrates a zone Ylr provided by method (2) within which a viewer can observe a proper 3D image (hereinafter, this zone will be referred to as "3D-viewable zone"). The display device shown in FIG. 23 includes a plurality of pixels, i.e., pixels 2001(r) for the right eye and pixels 2001(l) for the left eye, with a black matrix 2002 provided in regions not occupied by the pixels 2001(r) and 2001(l). A lenticular lens plate 2005 is disposed in front of the display device, the lenticular lens plate 2005 including cylindrical lenses 2006 at a pitch corresponding to every two pixels (i.e., one for the right eye and one for the left eye). Thus, the lenticular lens plate 2005 functions to allocate appropriate images for the right eye and the left eye of a viewer 2007.
In FIG. 23, the viewer 2007 can observe a proper 3D image when the left eye thereof is positioned within the range C-D and the right eye thereof is positioned within the range E-F. However, if the viewer's eyes are moved, e.g, so that one eye is positioned in the range D-E, the viewer 2007 can no longer see a 3D image because the eye positioned in the range D-E is in fact seeing a dark portion 2002 (i.e., a portion of the black matrix) present between pixels. Similar failures to produce a 3D image occur while one eye is in the range B-C or the range F-G. Furthermore, when the left eye is positioned in the range E-F and the right eye is positioned in the range G-H, an image in which the component right image and left image are reversed is produced to the viewer, instead of a proper 3D image. Thus, the principle of method (2) has the disadvantage of providing a 3D-viewable zone which is confined within a small distance equal to the interval between the eyes of the viewer 2007.
Furthermore, if the viewer's eyes move farther away from the range where the right and left images are reversed, they will reach areas where they will observe the proper 3D image once again (referred to as a "sub-lobe"). In the autostereoscopic method, such sub-lobes may be utilized to allow a few people to simultaneously enjoy the 3D image, but the 3D-viewable zone provided in each sub-lobe is also very narrow.
Next, the 3D-viewable zone along the front-back direction (i.e., how close to and far away from the image display device the 3D-viewable zone extends) is described with reference to FIG. 24. In FIG. 24, 1, 2, . . . 7, and 8 each denote a light beam exiting a pair of display pixels (right and left) at either side or the central portion of the display device. The 3D-viewable zone, having a front-back dimension and a right-left dimension, is indicated by the hatched portion in FIG. 24. The following is assumed in this exemplary case: the eyes of the viewer 2007 are located at an interval of e; the display device has a horizontal dimension Hh; and the size of the horizontal dimension (i.e., the dimension along the right-left direction) of the 3D-viewable zone is maximized when the display device is viewed at a viewing distance L. Under such conditions, the forward dimension Yf of the 3D-viewable zone (i.e., the distance along which the viewer 2007 can move forward from the position defined by the optimum distance L without losing the 3D image), and the backward dimension Yb of the 3D-viewable zone (i.e., the distance along which the viewer 2007 can move backward from the position defined by the optimum distance L without losing the 3D image) are expressed by equations (1) and (2), respectively, according to the equations described in NHK GIKEN R&D No.38, p.52 (eight-eye autostereoscopic television: ISONO, YASUDA, et al. pp.43-54, NHK GIKEN R&D, No.38, November 1995): EQU Forward dimension Yf of the 3D-viewable zone=e.times.L/(Hh+2.times.e)eq. (1 ) EQU Backward dimension Yb of the 3D-viewable zone=e.times.L/Hh eq. (2)
For example, the distances Yf and Yb along which the viewer 2007 can move forward or backward from the position defined by the optimum distance L without losing the 3D image are derived to be 67 mm and 109 mm, respectively, under the conditions that a TFT liquid crystal display panel having a diagonal dimension of 10.4 inches (i.e., with a vertical dimension Hv: 156 mm and a horizontal dimension Hh: 208 mm) is used as a display device; the viewer's eyes are located apart by 65 mm; and the display device is viewed at a distance L of 350 mm. If the viewer moves either closer to (i.e., forward) or away from (i.e., backward) the above-defined zone, the 3D image will be lost from the sight of the viewer 2007.
There is also a problem in that the 3D images provided in sub-lobes have poor image quality. In fact, Japanese Laid-Open Publication No.4-16092 discloses a method for eliminating sub-lobes by using light shielding plates, in order to solve this problem.
FIG. 25 schematically illustrates the concept of the method disclosed in the Laid-Open Publication No.4-16092. As shown in FIG. 25, this method employs a light shielding plate 2013 for restricting the viewing angle along the horizontal direction of the display screen, the light shielding plate 2013 being located in front of a lenticular lens plate 2012 having cylindrical lenses that are disposed at a predetermined period along the horizontal direction of the display screen. Each cylindrical lens extends in parallel to the vertical direction of the display screen. The light shielding plate 2013 includes a plurality of light shielding layers 2014 each extending along the vertical direction of the display screen for shielding any portion of light other than that from each appropriate pair of right and left pixels, thereby eliminating the above-mentioned sub-lobes. Thus, this method achieves 3D viewing only in the main lobe, which provides high image quality. However, this method limits the viewing of the 3D image in the range within the main lobe, and therefore the 3D viewable zone is not expanded.
Japanese Laid-Open Publication No.6-335030 discloses a 3D image display device including a masking means located between a lenticular lens plate and the display device so as to correspond to the light-intercepting portions (i.e., black stripes) of the display device, the masking means being capable of altering optical paths. As shown in the schematic conceptual diagram in FIG. 26, a diffusion plate 2026 serves as the masking means, having a plurality of diffusion layers 2022 and a plurality of masking layers present between the diffusion layers 2022. The diffusion plate 2026 is arranged so that the masking layers are located in front of non-displaying region 2025 (i.e., black stripes) present between each pixel for the right eye and each pixel for the left eye, so as to block portions of light from the non-displaying portions and to diffuse portions of light from the openings (pixels). Thus, this method substantially prevents black belts from being generated in the eight of the viewer. However, this method, while preventing the generation of black belts, does not expand the 3D-viewable zone because the diffused light makes a larger region susceptible to crosstalk.
Finally, method (3) allows a viewer to observe a 3D image without flickering problems by wearing polarizing glasses, which generally are very cheap and lightweight. However, this method requires two display devices or projector devices for simultaneously providing two images having different axes of polarization, inevitably resulting in high cost and therefore being unsuitable for home-oriented use.
A variant of method (3) intended to solve the above-mentioned problem is disclosed in Japanese Laid-Open Publication No.58-184929. According to this method, a mosaic pattern of polarizing layers such that the axes of polarization of adjoining layers extend perpendicularly to each other is placed in close contact on the front face of a single display device, with the viewer wearing polarizing glasses to observe a 3D image. As shown in FIG. 27, this method employs polarizing plates 2034a and 2034b having axes of polarization extending perpendicularly to each other, the polarizing plates 2034a and 2034b being disposed in front of the display surface of a CRT (cathode ray tube) having pixels 2031(r) for the right eye and pixels 2031(l) for the left eye. The viewer can observe a stereoscopic image by viewing the images displayed on the CRT through polarizing glasses 2035 with a right lens and a left lens having respectively appropriate axes of polarization.
Furthermore, Japanese Laid-Open Publication No.62-135810 discloses a system capable of producing a 3D image by employing a single display device along with polarizing layers including portions with different axes of polarization provided on the inside of a glass substrate of the liquid crystal display device. As shown in the schematic conceptual diagram in FIG. 28, the liquid crystal display device includes a pair of glass substrates 2041a and 2041b and a liquid crystal layer 2045 sealed therebetween. Wiring layers 2043a and 2043b for applying an electric field to the liquid crystal layer 2045 and alignment films 2044a and 2044b for aligning the liquid crystal molecules within the liquid crystal layer 2045 are provided on the glass substrates 2041a and 2041b. As seen from FIG. 28, the polarizing layers 2042a and 2042b having portions with different axes of polarization are provided between the wiring layer 2043a and the glass substrate 2041a, and between the wiring layer 2043b and the glass substrate 2041b, respectively.
As described above, the autostereoscopic method described in (2) above has a problem in that the 3D-viewable zone is restricted in its horizontal (i.e., right-left) width, as well as in its depth (i.e., front-back width).
The image display devices disclosed in Japanese Laid-Open Publication No.4-16092 and Japanese Laid-open Publication No.6-335030 can display a 3D image in the main lobe alone, thereby providing high image quality, or display a 3D image while reducing the occurrence of black belts due to the non-displaying portions. However, both of these devices employ a lenticular lens plate as a means for allocating images for the right eye and the left eye. Since a lenticular lens plate includes cylindrical lenses disposed so that each cylindrical lens corresponds to two pixels (i.e., one for the right eye and one for the left eye) arranged along the horizontal direction on the display screen. Therefore, in the case where a two-dimensional (2D) image is displayed by these image display devices to a viewer, the horizontal resolution of such a 2D image is 1/2 of the actual horizontal resolution that is inherent to the display device. Moreover, the horizontally alternate arrangement of the pixels for the right eye and the pixels for the left eye requires an accurately-timed alternate switching, e.g., at a period of (1 horizontal period)/(number of pixels along the horizontal direction), between the signals (i.e., one for the right eye and one for the left eye) which are supplied for the display device. This inevitably complicates the driving circuitry of the display device. Furthermore, eliminating sub-lobes as does the device disclosed in Japanese Laid-Open Publication No.4-16092 increases the difficulty for a large number of viewers to simultaneously observe a 3D image.
On the other hand, the stereoscopic image display device in accordance with method (3) (shown in FIG. 27) has the following disadvantages. As shown in FIG. 27, a glass substrate (face plate) 2033 is present between the pixels 2031(r) for the right eye and the polarizing plate 2034a for the right eye, and between the pixels 2031(l) for the left eye and the polarizing plate 2034b for the left eye. Therefore, a proper 3D image can be observed by a viewer situated orthogonally to the displayed image on the CRT. However, if the positions of the viewer's eyes shift along the vertical direction, the image for the right eye and the image for the left eye may reach the wrong eyes (such a phenomenon is referred to as "crosstalk"), so that a proper 3D image can no longer be obtained.
The "crosstalk" phenomenon in the above case is further described with reference to FIG. 29. FIG. 29 illustrates a 3D-viewable zone provide by a stereoscopic image display device of the structure shown in FIG. 27.
The 3D-viewable zone Yud which allows for the viewer's movement along the vertical direction without losing the proper 3D image can be expressed by the following equation (3): EQU Yud=B.times.L/d eq. (3)
where P denotes the pitch of the display pixels 2031; B denotes the width of the non-displaying portions (black stripes) 2032; L denotes the distance between the display element and a viewer 2037; and d denotes an adjusted thickness of the transparent substrate for a vacuum which is calculated from the refractive index and the actual thickness thereof.
As shown in FIG. 29, the viewer 2037 is free to move between J and K without losing the proper 3D image. Therefore, the front-back dimensions of the 3D-viewable zone can be expressed by the following equations (4) and (5): EQU Yf=Yud.times.L/(Hv+Yud) eq. (4) EQU Yb=Yud.times.L/(Hv-Yud) eq. (5)
where Yf denotes the distance along which the viewer 2037 is allowed to move forward from a position defined by the optimum observation distance L without losing the proper 3D image; Yb deiotes the distance along which the viewer 2037 is allowed to move backward from the position defined by the optimum observation distance L without losing the proper 3D image; Hv denotes the vertical (i.e., along the direction parallel to the signal lines) dimension of the display element.
In eq. (5), Yb takes a negative value when Yud.gtoreq.Hv, indicating that there is no limit to the backward dimension of the 3D-viewable zone.
For example, a case is considered where a TFT liquid crystal display panel having a diagonal dimension of 10.4 inches (i.e., with a vertical dimension Hv: 156 mm and a horizontal dimension Rh: 208 mm), a pixel pitch P of 0.33 mm, and black stripes with a width B of 0.03 mm is used as a display device. If a counter glass substrate 2033 of the liquid crystal panel has a thickness d1 of 1.1 mm and a refractive index n of 1.52, the counter glass substrate 2033 has an adjusted thickness of 0.72 mm (an actual thickness of 1.1 mm). Therefore, if the display device is designed so that the distance L from the display device to the viewer is 350 mm, the 3D-viewable zone Yud along the vertical direction is derived to be 14.5 mm according to eq.(3). In other words, along the vertical direction, the viewer can only move by about 7 mm up or about 7 mm down from the center of the display screen without allowing crosstalk to occur. Moreover, according to eq.(4) and eq.(5), the 3D-viewable zone along the front-back direction has the following dimensions: Yf=29.9 mm and Yb=36 mm. If the viewer moves either closer to (i.e., forward) or away from (i.e., backward) the above-defined zone, the 3D image is lost.
On the other hand,. as mentioned above, Japanese Laid-Open Publication No.62-135810 discloses providing a polarizing plate having portions with different polarization axes on the inside of a liquid crystal display panel in order to prevent crosstalk between the image for the right eye and the image for the left eye. However, this publication does not disclose any specific methods for producing such a device.
Moreover, in order to eliminate the crosstalk phenomenon when viewing a 3D image as described in this publication, it is necessary to place a polarizing plate on the inside of each one of the pair of substrates (i.e., a TFT substrate and a counter substrate) of the liquid crystal display panel, each polarizing plate having portions with different polarization axes. However, display modes which are currently adopted for commercial products, i.e., the TN mode and the GH mode, require an alignment film (for aligning liquid crystal molecules) to be disposed on the polarizing plate, thereby introducing the following problem:
In general, alignment films used in liquid crystal display panels of the TN mode or the GH mode are obtained by forming a thin film of an organic polymeric material precursor (e.g., polyimide) by spincoating followed by annealing at about 180.degree. to about 250.degree. for imidizing the thin film, or alternatively depositing an inorganic material such as SiO.sub.2 at about 200.degree.. Therefore, in either case, the polarizing plates will be exposed to a high temperature during the formation of the alignment films.
However, iodine or dye type polarizing plates composed essentially of an organic polymer material, e.g., polyvinyl alcohol (PVA), polarizing plates are not heat-resistant, and therefore may lose the orientation order of the molecules of iodine or a dye, thereby resulting in insufficient polarizing ability. Hence, it is presumable that when the 3D image display device disclosed in Japanese Laid-Open Publication No.62-135810 is implemented by utilizing the technique commonly employed in the art at present, the polarizing ability of the polarizing plate may deteriorate, thereby resulting in a low display quality.
Similarly, when employing an active matrix type liquid crystal display panel (AM-LCD) including thin film transistors (hereinafter referred to as "TFTs") formed thereon, the polarizing plate will be exposed to a high temperature such as about several hundred degrees centigrade during the process of forming the TFTS. Hence, for the above-described reasons, it is impractical to form polarizing plates composed of commercially available materials for forming polarizing plates before the TFTs are formed.